People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Children With Autism May Increase Siblings' Risk Of Developing The Disorder

Although the exact cause of autism remains unknown, a recent study has found evidence that the risk of a child developing an autism spectrum disorder (ASD) is 14 times higher if an older sibling has ASD.

Children with older siblings who have ASD have an ASD rate of 11.3 percent, compared to only 0.92 percent for children with unaffected older siblings. In addition, children born at term who had an older sibling diagnosed with ASD had more than 15 times the increased risk for a diagnosis than age-matched children without an older sibling with ASD. Younger siblings who were born preterm and had an older sibling diagnosed with ASD had an almost 10 times greater risk for ASD. Lastly, younger boys with ASD who had older brothers were much more likely to be affected by the disorder than younger girls with older sisters.

These findings are based on the results of the medical records of 53,336 children born from 2001 to 2010, of which 592 were diagnosed with ASD. The researchers focused on at least two siblings born to the same mother.

The reason for this association is not clear, but the researchers explained in a recent statement one theory — parents who have an older child with an autism diagnosis are more likely to have their younger siblings tested too. A similar diagnosis bias may also explain the different ASD rates between boys and girls.

"Previous research has indicated that girls may be less likely to be referred for a diagnostic assessment or, if referred, they may be more likely to be misdiagnosed than boys," said senior study author Darios Getahun in a statement.

While we aren’t sure what causes autism, scientists recently have been pretty clear in pointing out what isn’t. For example, for years it was believed that there was a link between autism and vaccines, but this connection was proved false. In addition, induced labor and C-section delivery were also shown to play no role in a child’s risk for developing autism.